1. Field of the Invention
This invention relates to a projection display device for modulating polarized luminous flux by an optical modulation means in accordance with a given image signal to display an image on a projection surface.
2. Description of Related Art
As a projection display device utilizing polarized luminous flux, one described in Japanese Patent Publication No. Hei 1-302385 has been known. FIG. 17 schematically illustrates the construction of the main part of a conventional projection display device. This projection display device comprises three liquid crystal panels (liquld crystal light valves) 21, 23, 25 which are optical modulation means, a cross-dichroic prism 30 which is color light synchronizing means, and a projection lens 40 which is a projection optical system. Red light, blue light, and green light are incident on the three liquid crystal light valves 21, 23, 25, respectively, and color light of three colors is modulated and emitted in accordance with given image information (image signal). The cross-dichroic prism 30 synthesizes and emits color light of three colors in the direction of the projection lens 40. The projection lens 40 projects light which represents a synthesized color image on a screen 50. More specifically, in the cross-dichroic prism 30, a red reflecting film 31 for reflecting red light only, and a blue reflecting film 33 for reflecting blue light only are formed almost crosswise. Therefore, the red light is reflected from the red reflecting film 31, the blue light is reflected from the blue reflecting film 33, and the green light is transmitted by the red reflecting film 31 and the blue reflecting film 33 to be emitted from the cross-dichroic prism 30. The color light of three colors emitted from the cross-dichroic prism 30 is projected on the screen 50 by the projection lens 40, so that a synchronized image of color light of three colors is projected.
Ideally the red reflecting film 31 and the blue reflecting film 33 totally reflect the respective color light and transmit all green light. Actually, however, they reflect green light to some extent, and transmit the color light which should be reflected to some extent. For example, as shown in FIG. 17, some blue light in the blue light emitted from the liquid crystal light valve 23 is reflected from the blue reflecting film 33 and the red reflecting film 31, and the reflected light (return light) BRB illuminates the emitting surface of the liquid crystal light valve 23. A modulation control element for controlling the modulation of light is placed on the side of the emitting surface of the liquid crystal light valve 23, and energy of the return light BRB causes an incorrect operation of the modulation control element. Therefore, there may be a case where an incorrect modulation of light is performed in the liquid crystal light valve 23, and an incorrect image is displayed.